The launch was delayed for two days by high winds  first at Cape Canaveral, where they threated the Atlas V rocket, and then, improbably, by a storm that knocked out power to the control center in Maryland. But the New Horizons mission finally lifted off successfully on Thursday, en route to a 2015 encounter with Pluto.

Given that a debate has been raging for the past few years over whether Pluto is even a planet, it might be tempting to wonder "why bother?" In fact, though, the debate underscores why it's an even more important mission than anyone might have imagined just a couple of decades ago. Back then, Pluto was considered a sort of one-of-a-kind oddball, a big chunk of mostly ice, very different from the rocky inner planets (like Earth) or the gassy outer ones (like Jupiter).

But over the past few years, astronomers have realized that Pluto is just an especially big  and not even the biggest  of a giant swarm of icy bodies, known collectively as the Kuiper Belt, which orbits the Sun out beyond Neptune. If it's just one among many, it either has to be downgraded from its original status as a full-fledged planet, or at least a handful of other big Kuiper Belt objects have to be added to the list. But in any case, these lumps of ice haven't melted since the origin of the Solar System some 4.4 billion years ago, and so, unlike the closer planets, they are a geochemical snapshot of the ancient past, which can tell astronomers an enormous amount about where we came from.

That's what New Horizons will be investigating when it reaches Pluto, its major moon Charon, and two smaller moons, found just last fall. And while a more than nine-year journey sounds like a long one, it's remarkably quick for a probe that has to travel more than 3 billion miles. Indeed, at a top speed of more than 47,000 m.p.h., which it will achieve by playing off Jupiter's gravity in a 2007 flyby, New Horizons will be the fastest spacecraft in history (it's no slouch even now: a mere nine hours after launch, it will zip past the Moon; it took Apollo astronauts three days to get that far).

Between now and 2015, though, the project scientists won't exactly be sitting on their hands. Says Alan Stern, Principal Investigator for the New Horizons mission and head of space sciences at the Southwest Research Institute in Boulder, Colo., "People ask me if I'll go into a depression now. But I have a job, and I'm working on lots of projects. Anyway, in a year it will be at Jupiter, and at that point we'll have a whole lot of data to process."